Recently, with the help of 3D seismic interpretation, the polygonal faults were discovered in Nenjiang Formation of Songliao Basin. It is the first discovery of polygonal fault in the eastern continental basins of China. By means of 3D seismic achievements, the characteristics of profile and plane of the faults are described. In the meantime, the forming origins of the faults are obtained in Nenjiang Formation in Songliao Basin by the origin model interpretations of overpressure syneresis. And furthermore the signifieances for the discovery are discussed.